Optical pointing devices are already known in the art. U.S. Pat. No. 5,288,993 for instance discloses a cursor pointing device utilizing a photodetector array and an illuminated target ball having randomly distributed speckles. U.S. Pat. No. 5,703,356 (related to the above-mentioned U.S. Pat. No. 5,288,993) further discloses (in reference to FIGS. 23A and 23B of this document) an optical cursor pointing device in the form of a mouse which does not require a ball and wherein light is reflected directly from the surface over which the pointing device is moved.
In both cases, the optical pointing device includes a light source for repetitively illuminating a surface portion (i.e. a surface portion of the ball or a portion of the surface over which the optical pointing device is moved) with radiation and an optical sensing unit comprising a photodetector array including a plurality of pixels each having a photosensitive element which is responsive to radiation reflected from the illuminated surface portion. The pixels outputs of the photodetector array are typically coupled to conditioning and processing circuits for tracking and extracting information about the relative motion between the sensing unit and the illuminated surface portion.
The technique used in above-cited U.S. Pat. Nos. 5,288,993 and 5,703,356 in order to extract motion-related information is based on a so-called “Edge Motion Detection” technique. This “Edge Motion Detection” technique essentially consists in a determination of the movement of edges (i.e. a difference between the intensity of pairs of pixels) in the image detected by the photodetector array. Edges are defined as spatial intensity differences between two pixels of the photodetector array. The relative motion of each of these edges is tracked and measured so as to determine an overall displacement measurement which is representative of the relative movement between the photodetector array and the illuminated portion of the surface.
An improved motion detection technique based on the above “Edge Motion Detection” technique is the subject matter of a pending international application No. PCT/EP 02/13686 filed on Dec. 3, 2002 (under priority of U.S. provisional application No.60/335,792 of Dec. 5, 2001) in the name of EM Microelectronic-Marin SA and entitled “Method and sensing device for motion detection in an optical pointing device, such as an optical mouse” (this international application is published under No. WO 03/049018 A1). The above international application describes various motion detection algorithms which are all based on a common basic assumption, i.e. that motion of the sensor with respect to the illuminated surface between two successive measurements is less than the pixel pitch, i.e. the spacing between adjacent pixels of the photodetector array.
For performance reasons, it is highly desirable to design the motion sensing part of the optical pointing device so as to exhibit a high detection resolution. On the other hand, a PC or external controller to which the detected displacement are to be reported does not require high resolution motion reports. An appropriate reporting scheme accordingly needs to be proposed so as to “shape” the motion reports to the resolution required by the PC or external controller. It is an object of the present invention to provide such a reporting scheme.